Turnover of cells and matrix occurs in a wide spectrum of organs and tissues and is essential to maintenance of tissue integrity. In bone, numerous investigators have suggested that a major function of osteonal remodeling is to maintain tissue, wherein remodeling serves to remove and replace microscopic regions of compact bone, which have become microdamaged due to fatigue. Left undetected and unrepaired, microdamage in bone leads to compromised mechanical properties and bone fragility. How bone remodeling units "target" microscopically damaged areas of bone remains a mystery. Previously, we found that osteocytes undergo apoptosis in bone areas immediately surrounding bone fatigue microdamage. This apoptosis precedes the onset of osteoclastic resorption. The resulting areas of osteocyte apoptosis co-localize exactly with the areas of bone, which subsequently undergo resorption by osteoclasts. These observations are the first demonstration of a potential cellular mechanism to explain how microdamage can initiate focal bone remodeling. In the current studies, we will determine whether osteocyte apoptosis is also highly associated with bone resorption stimulated by metabolic challenge (i.e. ovariectomy in rats) and mechanical challenge (i.e. immobilization in rats). Osteocyte apoptosis will be assessed by TUNEL and Bax expression in situ, and quantitated by histomorphometry. In the second series of studies, osteocyte apoptosis and targeted remodeling will be activated using ulnar bending to fatigue long bones in adult rats. Osteocyte apoptosis will then be pharmacologically modulated in vivo using a caspase inhibitor, in order to determine whether the extent of osteocyte apoptosis directly influences the activation, targeting and/or progression of osteoclastic resorption. In the third series of studies, we will examine changes in bone lining cells overlying regions of osteocyte apoptosis. We will determine whether lining cells also change their integrity in concert with underlying osteocytes, and thereby potentially present phagocytic markers to responding osteoclasts and precursors.